The invention relates more particularly to a molding unit for the forming of a container made of thermoplastic material by blow molding, particularly by stretch blow molding, of a preform, the molding unit having:                two half-molds that are transversely mobile relative to a fixed holder between a molding joined position and a separated position for extraction of the finished container;        a mold bottom that is vertically mobile between a lower position for extraction of the finished container and an upper position for molding of a base of the container, the mold bottom being able to be moved between the two half-molds in a molding joined position over a directed path, termed boxing, which extends from an initial intermediate boxing position to its upper molding position;        a carriage that carries the mold bottom and that is mounted to move in the fixed holder;        first actuation means to move the carriage relative to its fixed holder between a lower position of the mold bottom and its initial intermediate boxing position;        second actuation means having at least one cylinder housing a piston sliding vertically that is able to push the mold bottom over its boxing path by pressurizing a lower work chamber of the cylinder.        
Such mold units having a device to perform a so-called “boxing” operation of the base of a container, such as in particular a bottle made of thermoplastic material, are known in the state of the art.
Such a boxing operation is, for example, described in the document FR 2,508,004 to which reference will be made, in a nonlimiting way, for further details about this forming technique.
The boxing operation corresponds to a forming technique used during the manufacture of containers for which particular characteristics are sought, particularly of material with structural rigidity at the base thereof, for example for certain plastic bottles made of PET (polyethylene terephthalate).
This forming technique is particularly, but not exclusively, used for the purpose of manufacturing containers such as those bottles that are intended to be filled “hot,” i.e., filled with a liquid to be packaged whose temperature is, for example, between 60° C. and 95° C. It makes it possible to obtain a flexible base that forms a membrane that is deformed under the action of the partial vacuum created by the cooling of the hot liquid contained in the sealed container.
This technique is further able to be used to reduce the amount of material present in the base of the container whose weight it is desired to reduce so as to lower its cost and/or even for esthetic reasons.
The boxing technique consists in moving, during forming, the portion of the mold intended for the molding of the base of the container, called “mold bottom” in the following description, or even “mold of the bottom” in the previously referenced document. This movement is performed while the material that makes up the (formed or forming) preform or container still has a temperature appropriate for deformation. This movement makes it possible at the mold bottom to stretch the material that makes up the base of the container. The mold bottom thus rises like a fist to “box” the base of the container during forming, hence the name of the boxing operation.
In known molding installations that do not use a boxing operation, it is already known to move the mold bottom between its lower position for extraction of the container and its molding position by means of a cam actuation device. Such a device makes it possible in particular to obtain a rapid and precise control of the movements of the mold bottom. Such rapidity of actuation is not possible with a pneumatic cylinder. Nevertheless, the cam actuation devices do not make it possible to obtain enough force to perform a boxing operation.
To solve this problem, it has been proposed to control the movements of the mold bottom by means of a first cam actuation device that makes possible a rapid and precise positioning of the mold bottom, and by means of a second pneumatic actuation device that makes it possible to move the mold bottom over its boxing path with enough force to perform the boxing operation.
FIGS. 5 to 8 of the document FR-2,508,004 represent the main steps of this technique and illustrate an example of a boxing operation implemented during the forming of a bottle made of thermoplastic material.
By comparison with a conventional mold having two half-molds and a separate mold bottom, the mold bottom is not axially immobilized by the joined half-molds, but it is able to be moved axially by actuation means between an initial intermediate boxing position (low position) and a final upper molding position (high position).
In FIG. 5 that illustrates a first step, the mold bottom is moved toward the half-molds to occupy the initial position in which it is positioned axially retracted so as to ensure that the axial dimension (height) of the molding cavity is greater than that of the body of the final container.
A preform, introduced into the mold, extends axially through an opening coming out into a molding cavity that delimits the two half-molds associated with the mold bottom, said half-molds mounted to move between a separated position and a joined position (shown in FIG. 5).
In FIG. 6 that illustrates a second step, the preform is stretched axially by a stretching rod until reaching approximately the uppermost part of the mold bottom.
The mold bottom then still occupies its initial intermediate boxing position; the axial stretching of the preform is accomplished along a maximum stretching path that is greater than the height of the container (bottle), or with an over-travel corresponding to the difference between the initial intermediate boxing position and the final molding position of the mold bottom. In this final molding position, the axial dimension of the molding cavity corresponds to that of the final container.
Such a stretching operation is performed in combination with a blow-molding operation, preferably respectively preceded by a pre-blow molding, so as to obtain a bi-axial stretching, i.e., a bi-axial molecular orientation of the material whereby the structural rigidity of the base of the container in particular is improved.
At the completion of this second step, the base of the container here is not yet formed, and the material still has a deformation temperature. The mold bottom is then moved axially upward by associated actuation means, from the initial position up to the final molding position illustrated in FIG. 8.
The mold bottom travels an axial boxing path during which said mold bottom will stretch the material by axially pushing the base in the direction of the interior of the container.
However, according to the application, the techniques vary and the boxing operation is able to be accomplished during or after the forming of the container that is ultimately formed by a blow-molding operation at a blow-molding pressure.
The final blow-molding pressure can reach values on the order of 20 to 40 bars depending on the applications, or as a consequence of the values commonly considered as high pressures.
With the exception of the boxing operation, the forming of the container is consequently performed in a conventional manner by stretch blow molding of a preform made of thermoplastic material previously thermally conditioned for this purpose, with one or more blow-molding steps consisting in injecting a pressurized fluid, generally air, into the interior of the preform.
Of course, the description that has just been given is in no way limiting, and various variants or improvements can be made to it to perform the boxing operation.
To perform such a boxing operation of the base of the container during its forming, a device of the type described previously equips each container molding unit of the machine.
Such a molding unit has actuation means that are intended to move selectively the mold bottom between said initial intermediate boxing and final molding positions.
The document FR-2,945,469 describes an example of actuation means of such a molding unit that is able to perform a boxing operation.
According to this document, the actuation means of the mold bottom consist of a simple cylinder operating by means of a fluid under pressure formed by compressed air. The pneumatic cylinder is supported by a carriage that makes it possible to move the mold bottom from a lower extraction position to its initial intermediate boxing position. The cylinder is then actuated to move the mold bottom over its boxing path.
The device for performing the boxing according to this document and in particular the actuation means formed by such a cylinder, however, are not entirely satisfactory.
Actually, such a device is particularly cumbersome, and it does not make it possible to manufacture the containers in series with a rate as sustained as for containers manufactured without a boxing operation. This decline in rate is particularly due to the significant weight of the carriage thus provided.
The object of the invention is therefore to solve in particular the above-cited drawbacks and even to propose a new device design to perform the boxing operation that is advantageously simple, reliable, cost-effective, and especially rapid.